1. Field of the Invention
The present invention relates to a magnetic sensor using a magneto-resistance element, and a method of producing the magnetic sensor.
2. Description of the Related Art
In the related art, there are various kinds of sensors having magneto-resistance elements, and such kind of sensors include those using a magneto-resistance effect element (MR element), a magneto-impedance element (MI element), a fluxgate sensor, a semiconductor Hall effect sensor, and others.
For example, consider an MI sensor, since an MI element (a magneto-resistance element) is used, it is easy to make the MI sensor thin and small, and further improvement is being made.
As for an MR element, when a high-frequency current flow in the MR element, a high-frequency impedance of the MR element changes along with a magnetic field, and it is possible to detect the field strength of the magnetic field by utilizing the change of the high-frequency impedance caused by the magnetic field.
A Giant Magneto-Resistive element (GMR element) and a Tunnel Magneto-Resistive (TMR) element are well-known magnetic sensors using magneto-resistance (MR) effect elements.
A GMR element includes ferromagnetic layers and non-ferromagnetic layers stacked alternately, and detects the field strength of the magnetic field by utilizing a varying resistance occurring when the magnetization directions of two neighboring magnetic layers change between a parallel state and an anti-parallel state according to the strength of an external magnetic field.
A TMR element includes magnetic thin films stacked with insulating films in between. In the TMR element, electrons contributing to conduction penetrate the insulating film by a tunneling phenomenon while sustaining their spins; thereby, the tunneling coefficient changes along with the magnetization state in this process. The TMR element detects the field strength of the magnetic field by utilizing varying tunneling coefficients.
Such a magneto-resistance effect element (MR element) has a pinned layer (fixed layer) in which the magnetization direction is fixed in a specified direction, and a free layer in which the magnetization direction changes along with the direction of the external magnetic field. The magnetic sensor detects the direction of the external magnetic field by utilizing the fact that the resistance changes along with a relative relationship between the magnetization direction of the free layer changing along with the direction of the external magnetic field, and the magnetization direction of the pinned layer fixed during detection of the external magnetic field.
In recent years, devices using the GPS (Global Positioning System), such as car navigation devices and mobile phones, have become widely spread. Among these devices, in applications of confirming a current position in an area where electromagnetic waves from satellites of a GPS are shielded, very small magnetic sensors are required. For this purpose, a magnetic sensor formed on a silicon wafer and able to be integrated together with an IC (Integrated Circuit) is quite suitable. In addition, the magnetic sensor is required also in direction detection in a two-dimensional plane or in a three-dimensional space.
For example, Japanese Patent Gazette No. 3498737 (hereinafter, referred to as “reference 1”) discloses the magnetic sensor capable of the direction detection in the two-dimensional plane.
The magnetic sensor capable of the direction detection in the three-dimensional space is disclosed in, for example, Japanese Laid Open Patent Application No. 2003-008101 (hereinafter, referred to as “reference 2”), Japanese Laid Open Patent Application No. 2006-010591 (hereinafter, referred to as “reference 3”), and Japanese Laid Open Patent Application No. 2006-308573 (hereinafter, referred to as “reference 4”).
In reference 1, magneto-resistance effect elements are arranged in a plane of a substrate to be perpendicular to each other so as to detect changes of a magnetic field along two mutually perpendicular directions (for example, X direction, Y direction). In addition, in each of two directions, plural magneto-resistance effect elements are connected to form a Wheatstone Bridge circuit.
Reference 2 discloses a technique utilizing the TMR elements. In reference 2, one-axis TMR elements, each of which is able to detect variation of a magnetic field along one axis, are independently mounted along three axes perpendicular to each other by using a mounting technique to form a three-axis direction sensor.
In reference 3, a two-axis magnetic field detector and a one-axis magnetic field detector, which are formed from MR elements, are formed on a flexible substrate, and a three-axis direction sensor is obtained by bending the flexible substrate with a thin-film conductive member electrically connected to the two-axis magnetic field detector and the one-axis magnetic field detector.
In reference 4, magneto-resistance effect elements are arranged in a plane of a substrate to be perpendicular to each other so as to detect changes of a magnetic field along two perpendicular directions (for example, X direction, Y direction) further, magneto-resistance effect elements are arranged on an inclined surface formed on a substrate so as to detect changes of the magnetic field along a Z direction. In addition, in each of three directions, plural magneto-resistance effect elements are connected to form a Wheatstone Bridge circuit.
As described above, there are several kinds of three-axis magnetic sensors, such as the three-axis magnetic sensor disclosed in reference 2, which is formed by mounting one-axis magnetic elements in directions along three axes perpendicular to each other, and a three-axis magnetic sensor disclosed in reference 3, which is formed by arranging a two-axis magnetic field detector and a one-axis magnetic field detector on a flexible substrate, and bending the flexible substrate.
However, in the three-axis direction sensor disclosed in reference 2, because of the three-dimensional mounting, it is difficult to improve precision of the perpendicularity of the three axes, interconnection of electric wiring is complicated, and the three-axis direction sensor becomes relatively large.
In the three-axis direction sensor disclosed in reference 3, the precision of the relative positional relation of the magnetization directions of the two-axis magnetic field detectors, specifically, the magnetization directions of the pinned layers (fixed layer) in the axes, can be made very high. On the other hand, the precision of the relative positional relation of the two-axis magnetic field detectors relative to the one-axis magnetic field detector is determined by the way of bending the flexible substrate, and for this reason, the precision of the relative positional relation of the magnetization directions of the two-axis magnetic field detector and the one-axis magnetic field detector is lower than the precision of the relative positional relation of the magnetization directions of the two-axis magnetic field detectors. In addition, in order to bend and fix the flexible substrate, a certain overlap width is required, and this results in a thick and large substrate member for fixing the flexible substrate.
Uncertainty of the precision of the relative position between the three axes results in inclination of the position precision obtained from the measured output of the external magnetic field.
In addition, when installing the three-axis direction sensor in a mobile phone or other devices, small and thin magnetic sensors are required. However, the three-axis direction sensors disclosed in reference 2 and reference 3 have limitations in this aspect.
The direction sensor disclosed in reference 4 is proposed to solve the problems existing in the direction sensors disclosed in references 2, 3.
As described above, a magnetic sensor using a magneto-resistance effect element (MR element) has a pinned layer (fixed layer) in which the magnetization direction is fixed in a specified direction, and a free layer in which the magnetization direction changes along with the direction of the external magnetic field. The magnetic sensor detects the direction of the external magnetic field by utilizing the fact that the resistance changes along with a relative relationship between the magnetization direction of the free layer changing along with the direction of the external magnetic field, and the magnetization direction of the pinned layer fixed during detection of the external magnetic field. Therefore, the magnetization direction of the pinned layer (fixed direction) is different from an optimum direction in a multi-axis direction. The magnetization direction of the pinned layer is determined by thermal treatment at a given temperature in the magnetic field. For this reason, in two-axis or multi-axis sensors on the same substrate, for each axis, the magnetization direction is changed to magnetize the pinned layer (fixed layer), as disclosed in reference 1 and reference 4.